This invention relates to normally-closed thermally actuated cut-off links (also referred to commonly as thermal fuses, switches or cut-offs) of a type which responds to the ambient temperature surrounding the cut-off links by opening an electric circuit when the ambient temperature reaches a given control value. Such thermally actuated cut-off links, for example, are frequently physically incorporated into the windings of electric motors and in other devices requiring thermal protection and electrically connected in series with such devices so that the cut-off links will de-energize the devices involved when the ambient temperature exceeds a given safe value.
Ambient thermally actuated cut-off links have been manufactured in two different configurations, one of which is disclosed, for example, in U.S. Pat. No. 3,180,958 to P. E. Merrill, and the other of which is disclosed in U.S. Pat. No. 3,944,960 to Audette et al. In both of these types of cut-off links the ambient heat is transmitted to the interior of the link through a generally elongated cylindrically-shaped conductive casing initially closed at one end and open at the other end. A first power lead extends longitudinally into an insulating closure in the open end of the housing and terminates in a flat end making a separable contact interface with a spring metal connector member spring-urged thereagainst and having a plurality of contact-forming arm resiliently pressing against and making sliding contact with the conductive interior walls of the casing. A second power lead extends longitudinally into the closed end of the casing where it is crimped to or otherwise connected to the end wall of the casing to make a permanent inseparable low resistance engagement with the end wall. The interface of the contact-forming arms of the connector and the inside walls of the casing and the interface of the first power lead and the connector form two separable electric contacts between the power lead having a resistance much greater than than between the second power lead and the casing end wall. It is believed that at high rated currents of large electric motors or other devices requiring thermal protection heat develops at these separable contact interfaces which can appreciably affect the ambient temperature at which the link opens, which is lowered thereby.
In the type of thermally actuated cut-off link exemplified by the Merrill patent, the casing contains a sandwich of elements including a pellet of meltable material at the closed end of the casing, a first partially compressed spring, the contact-forming arm carrying connector urged against the end of the power lead passing through the open insulated end of the casing, and a second weaker partially compressed spring on the opposite side of the connector which applies a force to the connector in a direction tending to move the connector away from the power lead. When the pellet melts at the control temperature, the stronger spring expands until its force equals that of the weaker spring, and then the originally weaker spring expands to push the connector away from the end of the adjacent power lead to open the cut off link.
In the type of ambient thermally actuated cut-off links exemplified by the Audette et al patent, where deformable contacts are separated from an adjacent contact surface by an arm-deforming member (in a manner like that disclosed in an earlier U.S. Pat. No. 3,274,363 to McGirr et al), the sandwich of elements within the casing includes only a single partially compressed spring. This spring applies pressure against a meltable pellet, in turn, positioned contiguous to an arm-deforming member which, when the pellet melts, is pushed against the contact-forming arms of the connector to deform the arms inwardly away from the interior of the casing to open the fuse. In the types of cut-off links exemplified by the Merrill and Audette et al cut-off links described above, the constructions involved are such that the resistance of the contact interfaces described cannot be adjusted during or after assembly thereof, and differences in the internal resistance of what appear to be identical cut-off links, and creeping of the pellets thereof under prolong exposures to temperatures below but near the melting temperatures thereof, are believed to cause variations in the ambient temperature at which identical appearing fuses open.
There has been recently developed a normally-closed cut-off link which overcomes the aforesaid disadvantages of the prior art. This new normally-closed cut-off link comprises a cylindrical metal casing having a first power lead passing into and insulated from the casing, the power lead terminating in a pair of integral, resilient laterally outwardly inclining, deformable, contact-forming arms pressed against a backing member which expands the same against the inner surface of the casing. A second power lead is permanently connected, as by swaging, to the casing so that there is only one contact interface between the power leads, namely that between these arms and the casing. This contact interface is broken when the arms are contracted by a contact-deforming member on the inner sides of the contact-carrying arms and which is forced by spring pressure against the arms when the ambient temperature to which the link is subjected reaches a control temperature for which the link is designed. Additionally, the first power lead and contact-forming arms are preferably made of a relatively soft, very low resistance material, like silver coated copper, which, when pressed against the curved inner face of the casing, deforms somewhat to increase the contact area to minimize contact resistance.
The casing contains also a pellet of fusible material, preferably located at the initially closed end of the casing, a pair of opposed compressed spring means on opposite sides of the arm-deforming and backing members and a closure washer at the initially open end of the casing. The springs are held in a compressed state by the crimping of the casing around the closure washer while the springs are held compressed by external pressure applied to the washer. Upon the melting of the pellet, the arm-deforming member is forced by one of the springs against the contact-forming arms to bend them from the casing walls. Before the pellet melts, the force of the springs is not applied against the first power lead and associated contact-forming arms. Rather, these resilient arms are held in an expanded state against said backing member by the closure means of the casing which engage the first power lead.
After the various elements described have been inserted within the initially open end of the casing and prior to closing the open end thereof with a closure means, the first power lead and associated contact-forming arms are externally pressed inwardly toward the backing member with a progressively increasing force which spreads and forces the contact-forming arms progressively more firmly against the casing walls, until the measured contact resistance between the power leads drops to a predetermined desired value (like 0.9 milliohms when measured at 1.5" between probe points on the power leads). This adjusted force will generally provide a lower contact resistance with the casing wall than is readily achievable by the force of resilient contact-forming arms unaided by other forces, as in the above described prior cut-off links. When the contact resistance reaches this value, the power lead is anchored in its adjusted position by anchoring its closure means while it engages the first power lead. In its initially conceive form, the closure means was a curved body of epoxy material which covered and hermetically sealed the initially open end of the casing. The first power lead had one or more radial identations into which the epoxy material flowed, to aid in fixing the adjusted position of the power lead when the initially soft expoxy cement upon curing hardened.
The epoxy material forming this closure means was cured by placing the completed cut-off link in an oven heated to a desirable temperature (obviously below the desired melting temperature of the pellet of fusible material used in the cut-off link involved). The epoxy material curing process takes a relatively long period of time encompassing a number of hours and so it was necessary to maintain the adjusted external force on the contact-forming arm-carrying power lead until the curing operation was completed. This required the cut-off link-holding fixtures to remain attached to the cut-off links during the epoxy curing operation.
There was subsequently developed a different closure means making the manufacturing of the cut-off link more easy to carry out because the adjusted force on the contact-forming arms was fixed automatically upon the anchoring of a completed closure member (i.e. one not requiring any curing process to anchor the same) over the open end of the casing. This closure member comprised a preferably longitudinally split compressible resilient closure member which initially loosely enveloped the contact arm-carrying power lead extending into the casing of the cut-off link. After the power lead had been forced against the backing member so as to produce a desired measured contact resistance, the outer edges of the initially opened casing were crimped around the split closure member to compress the same tightly against the power lead, to fix the position of the power lead in the casing and to fix the pressure of the expanded contact-forming arm against the backing member and casing walls.
In our invention, an improved cut-off link construction utilizes a rigid closure member, for example, one made of a ceramic material and having an inner end which engages and is forced against a preferably laterally projecting portion of the contact arm-carrying power lead, to force the same with the desired pressure against the backing member. This rigid closure member is then fixed in position by crimping the casing around the end of the closure member.
In both forms of closure members just described, epoxy cement is applied to the lines of juncture between the closure member and the casing and first power lead, to hermetically seal these portions of the cut-off link. Since the closure member fixes the adjusted contact pressure of the cut-off link rather than the epoxy cement, the improved cut-off link can be placed in the epoxy-curing furnace without the necessity of any special pressure applying fixtures accompanying the same into the furnace.
The above-described and other objects and features of our invention will become apparent upon making reference to the specification to follow, the claims and the drawings.